Onlookers stand near a building called New Lucky House where a fire started in Hong Kong, on Wednesday. AP

A fire broke out in a residential building in Hong Kong's bustling Kowloon district on Wednesday, killing five people and injuring 19, the city's hospital authority and fire services said.

People inside the building waved towels at windows to signal for rescue, broadcaster RTHK said, adding that the fire had started at a gym there. Some surrounding roads had to be sealed off, disrupting traffic in the morning rush hour.

The blaze, rated three in the city's five-grade system, was doused by 8:54 a.m. (0054 GMT), an hour after fire services were alerted to the fire in Jordan, an area teeming with homes and offices across the harbour from the city's financial centre.

At least six of the injured were in serious condition, the hospital authority said, with one critical.

Fire fighters were still battling on Wednesday to put out a separate massive blaze 15 hours after it erupted at a building site in the northern New Territories, a fire services spokesperson said, but no casualties were reported.

Reuters